This invention relates to a method and apparatus for rejecting nitrogen from a feed gas stream comprising methane and nitrogen so as to form a methane product.
It is known to extract natural gas from underground reservoirs. The natural gas often contains nitrogen. The nitrogen may be in part or totally derived from nitrogen which has been injected into the reservoir as part of an enhanced oil recovery (EOR) or enhanced gas recovery (EGR) operation. A feature of such operations is that the concentration of nitrogen in the natural gas tends to increase with the passage of time from about 5% by volume to about 60% by volume.
U.S. Pat. No. 4,415,345 discloses a process for rejecting the nitrogen from the methane in a double rectification column operating at cryogenic temperatures. A double rectification column comprises a higher pressure rectification column, a lower pressure rectification column, and a condenser-reboiler placing the top of the higher pressure rectification column in indirect heat exchange with a region, usually the bottom, of the lower pressure rectification column. In the process according to U.S. Pat. No. 4,415,345 a stream of a mixture of nitrogen and methane is cooled at elevated pressure to a temperature suitable for its separation by rectification. A part of the feed gas is liquefied. The resulting gas mixture is separated by rectification. In one embodiment described in U.S. Pat. No. 4,415,345 a double rectification column is employed to carry out the separation. A liquid methane product is withdrawn from the bottom of the lower pressure rectification and is raised in pressure by a pump.
The methane product is typically required at a similar pressure to that at which the natural gas is supplied, for example, typically in the order of 40 bar. With relatively high methane feed purity in the order of 95% it is possible to pump the liquid methane product to about 25 bar upstream of its vaporisation which is effected by indirect heat exchange with the incoming feed gas. The vaporised product methane may be raised further in pressure by compression.
As the mole fraction of methane in the feed gas decays and the mole fraction of nitrogen in it rises, efficient heat exchange between the feed gas stream and the product methane stream can be maintained only at lower product stream pressures. For example, if the purity of the feed gas falls to 40% methane, the product methane stream needs to be vaporised at a pressure of about 9 bar. Difficulties arise in providing a compressor or series of compressors that is able to operate efficiently when its inlet pressure varies within such a wide range of pressures.
It is an aim of the present invention to provide a method and apparatus which ameliorates such difficulties.